If saving money is the primary objective, your friends can take the tube from Heathrow to Victoria (4 GBP/person) and then the Gatwick Express (up to four for the price of two special = 29.80 GBP divided by 3) from there to the airport. They can save even more money by booking DaySave tickets on a Southern train at least seven days in advance at www.southernrailway.com/main.php?page_id=194. Up to four for a total of 20 GBP. The ride from Victoria to Gatwick is only ten minutes slower than the Gatwick Express.